The present invention relates to a fibrous composite material for use in contact with fused aluminum or aluminum alloys.
Conventionally, the shell of a molten metal container and a guide tray for use in melting and casting of aluminum or its alloys, is made of an iron plate and its inner walls are lined with unshaped refractories such as castable or plastic refractories, or high alumina content bricks. The refractories protect the iron plate from an attack by the chemically active aluminum melt, from deposition associated with the attack, and from the iron plate contaminating the aluminum melt.
Refractory bricks or unshaped refractories which are widely used as lining refractories, generally have satisfactory heat-insulating properties but suffer from the disadvantage of high thermal conductivity (1.1-1.5 kcal/mh.degree.C. for high alumina content bricks and 0.5-1.2 kcal/mh.degree.C. for castable refractories). When the aluminum melt comes into contact with such refractories, the temperature of the aluminum melt seriously drops. In response to this disadvantage, energy saving heat-insulating boards or fibrous heat-insulating materials have received increased attention in recent years.
The inorganic fibers which are generally used in production of fibrous heat-insulating materials include SiO.sub.2 -based fibers such as alumino silicate, silica, glass, and mineral wool, alumina fibers, and zirconia fibers. Among others, alumino silicate-based ceramic fibers are widely used since they have a low bulk density, are light-weight, exhibit a great heat-insulating effect, are superior in antispalling properties, are excellent refractories and therefore are energy-saving and economically advantageous.
It is widely recognized that the alumino silicate-based ceramic fibers have many advantages in heat-insulation applications. They have, however, poor chemical stability when used in applications where they contact directly with an aluminum melt. The article "Effect of Molten Aluminum on Alumini-Silica Refractories", J. Am. Ceram. Soc., 35, 5, (1953), notes that alumina-silica-based refractories (bricks) are easily attacked by fused aluminum. This is also true of alumino silicate-based ceramic fibers. That is, when the ceramic fiber comes into contact with an aluminum melt, the SiO.sub.2 component of the ceramic fiber is reduced as indicated by the equation below and the Si formed is dissolved in the melt, resulting in gradual deterioration of the ceramic fiber. EQU 3SiO.sub.2 +4Al.fwdarw.3Si+2Al.sub.2 O.sub.3
Particularly in the case of molten aluminum alloys, the above reaction occurs markedly. As the reaction proceeds, deteriorated areas of the ceramic fiber form a dense modified layer (attacked layer), causing melt contamination and also attachment of the ground metal. The attached ground metal causes various problems, for example, the lining is damaged when attached ground metal is removed. Moreover, if part of the dense mass in the attacked areas is present in the melt, an inclusion is formed causing the formation of defects in casts. When the casts are rolled, metallic rolling members may be damaged. Thus, if a composite material containing alumino silicate-based ceramic fibers is to be used in contact with an aluminum melt, it is essential that the ceramic fibers be improved to resist corrosion.
A method of obtaining a material exhibiting a high resistance to permeation of an aluminum melt and an attack by the permeated aluminum melt is described in "How Molten Aluminum Affects Plastic Refractories", J. Metals, 35-37, January (1958). In the article, a clay compound mainly of kaolinite, a water-containing aluminum silicate, is subjected to a heat treatment and is used in the state of metakaolin. Based on the same technical concept, Japanese Patent Publication No. 17147/83 discloses a fibrous composite of kaolin clay and ceramic fibers. In this case, the composite is molded, dried, processed, and further calcined at elevated temperatures, thereby yielding the desired product.
In the case of the kaolin clay however, the degree of shrinkage involved in dehydration and transformation during the process of drying and calcination is large. For the composite of kaolin clay and ceramic fibers, the degree of shrinkage reaches about 2 to 5%. Thus, such a kaolin clay/ceramic fiber composite suffers from the limitation that it can be supplied only in the form of a molded article which has been subjected to a heat treatment at elevated temperatures of from 800.degree. to 1,800.degree. F. which are necessary to attain the formation of a metakaolin phase and its dimensional stabilization. Moreover, in cases where the composite is molded in complicated forms such as in the form of a pouring ladle or a distributor, much labor and difficulty are encountered in the molding operation. This disadvantage also occurs when the composite is applied to a heat-insulating board. Therefore, as a lining in a complicated form, the above-described castable or plastic refractory is usually used and applied in situ.
One object of this invention is to provide a fibrous composite heat-insulating material having improved performance.
Another object of this invention is to provide a fibrous composite material which will resist corrosion when in contact with an aluminium melt.
Another object of this invention is to provide a fibrous composite material which has minimal heat shrinkage and can be applied in situ.